Sensor for measuring an ion concentration or gas concentration

ABSTRACT

The invention relates to a sensor for measuring ion concentration or gas concentration, with a gas- or ion-sensitive layer ( 6 ) that has two sensitive partial areas ( 20  and  21 ), two conducting areas ( 1   b  and  2   b ), each coupled capacitively through air gaps ( 9 and  10 ) with one of the sensitive partial areas ( 20  and  21 ), with the capacitances of the couplings being different, and a comparison circuit ( 1   a   , 2   a   , 3, 4, 5 ) which has at least one first transistor (T 1 ) connected with the first conductive area and a second transistor (T 2 ) connected with the second conductive area, and at least one output at which a signal can be tapped that depends on the potential of sensitive layer ( 6).

BACKGROUND OF THE INVENTION

[0001] The invention relates to the field of sensors for measuring an ion concentration or gas concentration.

[0002] Sensors with field effect transistors (FETs) that have an ion-sensitive layer used as a gate are used to measure ion concentrations, with the potential of the layer depending on the ion concentration of a surrounding fluid or gas. For example, U.S. Pat. No. 5,911,873 shows such an ion-sensitive FET (ISFET). In addition, sensors with FETs are known for measuring gas concentrations, for example from U.S. Pat. No. 4,411,741, which have a gas-sensitive layer used as a gate, whose work function depends on the surrounding gas concentration.

[0003] Such sensors are generally produced from a drain and a source in a semiconductor substrate by counterdoping, and an insulating layer is grown or deposited on the substrate between the source and the drain. An ion-sensitive layer can be applied directly to this insulating layer. A gas-sensitive layer called a suspended gate FET (SGFET) can be applied at a certain distance. Alternatively, a gate can be applied to the insulator that is controlled capacitively by a gas-sensitive gate applied at a certain distance. This type of sensor is referred to as a capacitive-controlled FET (CCFET), and is described for example in German patent document DE 43 33 875 C2.

[0004] In these sensors the charge change caused by the ions to be detected or the change in work function caused by the gas molecules to be detected, is sensed as a change in the gate source voltage and the change in the drain source current caused thereby. The SGFET and CCFET have the advantage that the transducer formed from the substrate, drain, source, and insulating layer can be made independent of the sensitive layer.

[0005] A problem with these sensors is their temperature sensitivity. For example, the change in current caused by a temperature change of about 0.5 Kelvin may be greater than the signal change caused by the gas to be detected. In order to compensate for this temperature dependence, it is known that a second identical FET can be formed with a gate that does not have the gas-sensitive layer of the first gate and serves as a reference FET. One disadvantage of this arrangement is that two different gates must be applied in a small space. Another disadvantage is that the two FETs must be located relatively far apart and as a result can have different temperatures. With two different gates, the temperature pattern of the work function is generally different, so that good temperature compensation cannot take place.

[0006] Therefore, there is a need for a sensor for measuring an ion concentration or a gas concentration with reduced temperature dependence.

SUMMARY OF THE INVENTION

[0007] Briefly, according to an aspect of the invention, a sensor for measuring an ion concentration or a gas concentration couples two sensitive partial areas of a sensitive layer with different capacitances in a comparison circuit. The comparison circuit can be spatially separate from the sensitive layer. The spatial separation is achieved using process technology and can be formed in a small space and therefore with improved thermal coupling of its components over known measuring circuits. The comparison circuit can have two field effect transistors (e.g., MOSFETs) whose gates are connected with both conducting areas and whose source is common. The different control of the transistors measures the change in potential in the sensitive layer. Temperature variations cause the same changes in the transistor properties and result in negligible changes in the measured values.

[0008] Any conducting area coupled capacitively with the sensitive layer is applied with the gate of the related MOSFETs of the comparison circuit as a uniform transparent conductive layer, so that additional contacts are not necessary.

[0009] These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

[0010]FIG. 1 is a top view of a sensor according to one embodiment of the invention;

[0011]FIG. 2 is a section through the sensor in FIG. 1 along line A-A′;

[0012]FIG. 3 shows an electrical diagram of the sensor in FIGS. 1 and 2; and

[0013]FIG. 4 shows a section according to FIG. 2 through a sensor according to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014]FIG. 1 illustrates a sensor 100. The sensor includes a first n-doped drain 3 and a second n-doped drain 4 as well as an n-doped source 5 are formed in a second section 18 of a substrate 11 formed here as a silicon p-substrate by ion implantation. A thin oxide layer is applied between the first and second drains 3, 4 and the source 5, which for example can be about 8-20 nm thick and serve as a gate dielectric. A transducer is formed by the first and second drains 3, 4, the common source 5, and the thin oxide layer.

[0015] A thick oxide area with a first thick oxide layer 7 offset in one lengthwise direction with respect to this transducer and a thinner second thick oxide layer 8 that is offset laterally, are formed in a first section 17 of the substrate 11, as shown in FIG. 2. A first conductive layer 1 and a second conductive layer 2 that are offset laterally from one another, are applied to the thin oxide layer and the thick oxide layers 7 and 8, which however are not formed necessarily symmetrically in these embodiments, especially in mirror symmetry to one another.

[0016] Conductive layer 1 has a first conductive layer 1 b applied to first thick oxide layer 7 and an area that serves as first gate la applied to the thin oxide layer between first drain 3 and source 5.

[0017] Correspondingly, the second conductive layer 2 has a second conductive layer 2 b applied to second thick oxide layer 8 and an area serving as a second gate 2 a applied to the thin oxide layer between the second drain 4 and the source 5. The first and second conductive layers 1, 2 can be applied as poly layers or as metal layers. Two MOSFETs T1 and T2 are formed in second section 18.

[0018] A gate 6 with a sensitive layer is applied on additional intermediate layers 13, 15, and 16; the work output of the layer depends on the surrounding gas concentration. An area 22 is formed below the sensitive layer 6, which has a thinner air gap 9 between a first sensitive partial area 20 of the sensitive layer 6 and first conductive area 1 b and a thicker second air gap 10 between a second sensitive partial area 21 of the sensitive layer 6 and the second conductive area 2 b. According to the equivalent diagram in FIG. 4, this air gap acts as capacitors C1 and C2. Accordingly, the conductive areas 1 b and 2 b operate with thick oxide areas 7 and 8 and the substrate as capacitors C3 and C4.

[0019] For manufacturing, the first and second conductive layers 1, 2 are first applied to the thick oxide area and the thin oxide layer and structured using a mask process followed by etching. Then intermediate layers 13, 15, and 16 are deposited, structured, and etched over the conductive areas 1 b and 2 b so that these are exposed. In addition, the space for the subsequent air gap is formed through which gas exchange can occur beneath the sensitive layer.

[0020] In the embodiment in FIG. 4, an insulating layer 14 is formed on the conductive areas 1 b and 2 b, in contrast to the embodiment in FIG. 2. They can be applied together with the intermediate layer 13 and, in contrast to the embodiment in FIG. 2, are not removed by subsequent etching. The sensitive layer 6 can be applied directly to this layer as an ion-sensitive layer. By virtue of such an arrangement, ion concentrations in liquids and gases can be measured.

[0021] These embodiments can also be formed with the charge carrier types reversed.

[0022] Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention. 

What is claimed is:
 1. Sensor for measuring an ion concentration or a gas concentration with a sensitive layer (6) whose potential depends on a surrounding gas concentration or ion concentration, with the sensitive layer (6) having a first sensitive partial area (20) and a second sensitive partial area (21), a first conductive area (1 b) that is capacitively coupled by a first air gap (9) with first sensitive partial area (20), a second conductive area (2 b) coupled capacitively through a second air gap (10) with second sensitive partial area (21), with the capacitances of the two capacitive couplings being different, a comparison circuit (1 a, 2 a, 3, 4, 5) which has at least a first transistor (T1) connected with the first conducting area, and a second transistor (T2) connected with the second conducting area, and at least one output from which a signal can be tapped, depending on the potential of sensitive layer (6).
 2. Sensor according to claim 1, characterized in that air gaps (9, 10) are of different sizes.
 3. Sensor according to claim 2, characterized in that first conducting area (1 b) is coupled capacitively by a first dielectric layer (7), preferably a first thick oxide layer, and second conductive area (2 b) is connected by a second dielectric layer (8), preferably a second thick oxide layer, with a substrate (11).
 4. Sensor according to claim 3, characterized in that first dielectric layer (7) is thicker than second dielectric layer (8) and first air gap (9) is smaller than second air gap (10).
 5. Sensor according to claim 4, characterized in that the two sensitive partial areas (20, 21) are the same size and the two conductive areas (1 b, 2 b) are the same size.
 6. Sensor according to claim 5, characterized in that the comparison circuit has at least a first and second field effect transistor (T1 and T2), preferably MOSFETs, on a common substrate (11), so that a first gate (1 a) of the first field effect transistor is connected with the first conductive area (1 b) and a second gate (2 a) of the second field effect transistor is connected with second conductive area (2 b).
 7. Sensor according to claim 6, characterized in that the two transistors (T1 and T2) have a common source area (5).
 8. Sensor according to claim 7, characterized in that first conductive area (1 b) and first gate (1 a) are made as a first conductive layer (1) and second conductive area (2 b) and second gate (2 a) are made as a second conductive layer (2).
 9. Sensor according to claim 8, characterized in that first and second conductive layers (1, 2) are formed symmetrically, preferably in mirror symmetry to one another.
 10. Sensor according to claim 9, characterized in that first and second air gaps (9, 10) are formed as partial areas of a chamber (22) formed beneath sensitive layer (6).
 11. Sensor according to claim 10, characterized in that sensitive layer (6) is a gas-sensitive layer (6) and conducting areas (1 b, 2 b) are open to air gaps (9, 10).
 12. Sensor according to claim 11, characterized in that sensitive layer (6) is an ion-sensitive layer (6), applied to an insulating layer (13, 14), which extends over first and second conducting areas (1 b, 2 b).
 13. Sensor according to claim 12, characterized in that sensitive gate layer (6) and conducting areas (1 b and 2 b) are formed in a first section (17) of a substrate (11), and the comparison circuit is formed in a second section (18) of substrate (11) spaced lengthwise from first section (17).
 14. Sensor according claim 13, characterized in that conductive layers (1 and 2) are poly layers or metal layers.
 15. Method for making a sensor, in which an insulating layer (7, 8), preferably a thick oxide layer, is formed on a substrate (11) of a first charge carrier type in a first section (17) and in a separate second section (18), separated lengthwise, at least two drain areas (3, 4) and a source area (5) of a second charge carrier type are formed preferably by ion implantation between which a thin oxide layer is applied, a first and second conductive layer (1, 2) are each applied to insulating layers (7, 8) and the thin oxide layer in such a way that two conductive areas (1 b, 2 b) are formed in the first section above the insulating layer and two gates (1 a, 2 a) are formed in a second section, at least two air gaps (9, 10) are formed between insulating intermediate layers (13, 15), and a sensitive layer (6) is applied to the intermediate layers in such a way that air gaps (9 and 10) are formed between two partial areas (20, 21) of sensitive layer (6) and conducting areas (1 b and 2 b).
 16. Method according to claim 15, characterized in that conductive layers (16) are located between intermediate layers (13 and 15). 